Intense Laser Plasma Interactions on the Road to Fast Ignition

Linn D. Van WoerkomThe Ohio State University

APS DPPOrlando, FL

14 November 2007

FSC

Collaborators

D. Hey

F.N. Beg, T. Ma, S. Chawla, T. Bartal, M.S. Wei, J. King,J. Pasley

R.B. Stephens, K.U. Akli

R.R. Freeman, E. Chowdhury, D.W. Schumacher, D.T. Offermann, A. Link, V.M. Ovchinnikov

A.J. MacKinnon, A.G. Macphee, M. H. Key, H. Chen, R. Town, M. Foord, S. P. Hatchett, A.J. Kemp, A. B. Langdon, B. F. Lasinski, P. K. Patel, M. Tabak, T.H.Phillips

C. Chen, M. Porkolab, MIT, USA

R. C. Clarke, P. Norreys, D. Neely, RAL, UK

H. Habara, R. Kodama, H. Nakamura, K. Tanaka, T. Tanimoto U. Osaka, Japan

Y. Tsui, University of Edmonton, Alberta, Canada

Funding

• Office of Fusion Energy Science (OFES) – Advanced Concept Exploration Program

• Fusion Science Center (FSC)

• U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48

FSC

Toward Fast Ignition Point Design

Cone angle?

Electron source divergence full angle, s

fuel acceptance full angle, f

Laser input

electronsCompressed fuel

Optimal values from Atzeni (PoP 6 3316 1999); Atzeni (PPCF 47 B769–B776 2005); Tabak et al. (FS&T 49 254 2006)

• Laser intensity ~ 1020 W/cm2

• Pulse duration ~ 10-20 ps

• Cone tip ~ fuel size ~ 40 m

Revisiting Fundamental Issues

• We must revisit fundamentals for FI Point Design• Understanding the Electron Source for FI

– How many electrons w/ desired energy?How many electrons w/ desired energy?• Maximize efficiency of laser to electrons in 1-2 MeV range

• Must characterize the internal electron distribution

• What is internal Thot and how does it scale with laser intensity?

– How do we get the laser in?How do we get the laser in?• Cones

• Light guiding?

• Electron guiding?

Laser to Electron Efficiency

1.00E+08

1.00E+09

1.00E+10

1.00E+11

1.00E+12

1.0E+17 1.0E+18 1.0E+19 1.0E+20 1.0E+21

Peak Intensity (W/cm2)

Ab

so

lute

Yie

ld (

ph

/J/s

r)

25 um Cu,focused25 um Cu,defocus10 Al/30 Cu

10 Al/30 Cu/1000Al25 Cu + Plastic

April cones

Wolfgang

August cones

August 75degrees

Opacity corrected: includes most data from April and cones from August

Cu K yields measured by single hit/ HOPG

• Absolute K yields for Cu foils consistent with RAL PW data [Theobald et al., Phys. Plasmas 13, 043102 (2006)]

• Cones have yield consistently higher than slabs: Yield ~ constant with Intensity

Single Pass vs Refluxing Targets

• Single pass non-refluxing targets seem consistent with models

• 15-40% as laser intensity increased from 1018 to~ 1020 Wcm-2 • Refluxing targets seem to require constant 10%

e-

Single Pass target

What about ion losses??

Refluxing target

e-

Al Cu

Basic Conversion must be the same so analysis is incompleteBasic Conversion must be the same so analysis is incomplete- single pass needs Ohmic energy loss- refluxing needs Ohmic and fast ion corrections BOTH will BOTH will

increase inferred increase inferred efficiencyefficiency

What about the energy of the electrons?

• Many discussions regarding so-called TMany discussions regarding so-called Thothot

• Most measurements from vacuum electronsMost measurements from vacuum electrons– Only a very small fraction of electrons escape to vacuum

– Do these represent the internal distribution? (King, JO6.00011)

– Must include effects of time varying sheath potentials

• Bremsstrahlung measurements are trickyBremsstrahlung measurements are tricky– Closer representation to internal electron distribution

– BUT K-edge spectroscopy fails for Ephoton > 1 MeV

• Interpreting Data is key difficultyInterpreting Data is key difficulty– How do external measurements match to internal distributions?

– Figure of merit depends on application• Fast Ignition - # electrons w/ 1.5 < E < 2.5

• Protons – average energy

Revisiting Vacuum Electrons

RAL Magnetic SpectrometerRAL Magnetic Spectrometer

0 10 20 30 40 50MeV

~3mspectrometer

spectrometer

~.53 m

Titan Magnetic SpectrometerTitan Magnetic Spectrometer(from Chen et al. RSI 77 10E703 2006)

1000 2000 3000 40001E9

1E10

1E11

Aug 27 S10.5x0.5mm 25um thick CuTarget Normal127 J

Ele

ctr

on

s/M

eV

/sr

Energy (keV)

SigElectSr

Eaverage~ 1 MeV

• Complete spectrum is complicated …. Requires much more work• HOW do we interpret such spectra?

spectrometer

What is Thot?

• All roughly consistent with “TAll roughly consistent with “Thothot” near 1-2 MeV” near 1-2 MeV– Internal Distribution MeasurementsInternal Distribution Measurements

• Bremsstrahlung ~ 1 MeV (Chen, GP8.00056)

• Cone-wire analysis ~ 1 MeV (King, JO6.00011)

– External Distribution MeasurementExternal Distribution Measurement• Vacuum electrons ???? (Link, GP8.00064)

• Vacuum electron measurements in FI relevant energy region are not understood.

• More work needed to understand details for vacuum electrons

Revisiting Fundamental Issues

• We must revisit fundamentals for FI Point Design• Understanding the Electron Source for FI

– How many electrons w/ desired energy?How many electrons w/ desired energy?• Maximize efficiency of laser to electrons in 1-2 MeV range

• Must characterize the internal electron distribution

• What is Thot and how does it scale with laser intensity?

– How do we get the laser in?How do we get the laser in?• Cones

• Light guiding?

• Electron guiding?

How do we get the laser in? Cones

• Cone Cone willwill be used – keeps path clear for ignition laser be used – keeps path clear for ignition laser• What does the cone do?What does the cone do?

– Guide electrons?Guide electrons?• Surface magnetic field guiding electrons along preformed plasma -

Sentoku et al., PoP,11, 3083,(2004) Habara et al. PRL,97, 095004 (2006)

• BUT Recent Titan Ka measurements on oblique foils indicate no electron guiding

– Stephens, GP8.00043

– Guide light?Guide light?• Nakatsutsumi et al., PoP, 14 050701 (2007)• Nakamura et al., PoP, 14 103105 (2007)

• What is the role of preplasma?What is the role of preplasma?– Baton et al.

Oblique incidence yield lower

1.00E+08

1.00E+09

1.00E+10

1.00E+11

1.00E+12

1.0E+17 1.0E+18 1.0E+19 1.0E+20 1.0E+21

Peak Intensity (W/cm2)

Ab

so

lute

Yie

ld (

ph

/J/s

r)

25 um Cu,focused25 um Cu,defocus

10 Al/30 Cu

10 Al/30 Cu/1000Al25 Cu + Plastic

April cones

Wolfgang

August cones

August 75degrees

Opacity corrected: includes most data from April and cones from August

Vary angle of

incidence

spectralon

20080824

s2

20070830

s04

=28o

=75o

More reflected light for obliqueLess absorption

75o foils

Getting the light in….

• Tight laser focus on the tipTight laser focus on the tip– Even slightly messy focus gets there

•We have measured & modeledWe have measured & modeled• Defocus behind or inside the cone• Look at role of reflections• Use realistic absorption vs angle of incidence

Focus behind Focus inside

Measuring Cu Kemission

20070823s03

Tight focus at tip

• Wire grid figure is original cone projected through the imaging system including all view angles. “Hat brim” flange uniquely fixes geometry ….

• Using known distances there are no adjustable parameters

Cu Ka @ 8047 eV imaging using a spherical crystal Bragg mirror

Cu Ka Imaging in Cones

0 200 400 600 800

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

K

em

issi

on

(a

.u.)

Distance from cone tip (m)

s01

Cu ConesCone tip 30 m diameterCone walls 25 m thick

20070504s01 20070504s02

0 200 400 600 800-0.2

0.0

0.2

0.4

0.6

0.8

1.0

K

signal (

a.u

.)

Distance from cone tip (m)

s02

Tight focus aligned on cone tip

Light Guiding in Cones

0 200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

K

Sig

na

l (a

.u.)

Distance from cone tip (m)

s04

0 200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

K

Sig

na

l (a

.u.)

Distance from cone tip (m)

s05

0 200 400 600 800

0.0

0.1

0.2

0.3

0.4

0.5

0.6

K

Sig

na

l (a

.u.)

Distance from cone tip (m)

s06

20070504s04 20070504s05 20070504s06

400 m behind 400 m inside 800 m behind

All cone shots are basically the same ….

0 200 400 600 800 1000

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Cu

K s

ign

al (

arb

un

its)

position from tip of cone (microns)

s01 s02 s04 s05 s06

50 m rise 140 m extent

140 m exponential fall

All curves normalized to peak emission

Ray Tracing for Perfect Titan Beam

400 mSecond bounce and higher all have angles of incidence > 45o

• f/3 focused 400 m behind cone tip

• Even defocused beams are collected

• Without absorption the beam reflects back out

Absorbed Energy

400 microns behind tip

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.025 0.05 0.075 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5

avg distance from tip (mm)

relative no. of rays

total absorption

rel. half energy/area visible

•Light IS guided to the tipLight IS guided to the tip•Energy is absorbed in walls Energy is absorbed in walls •Closed end reflects light backClosed end reflects light back

~

• Absorption from Shepherd et al. from LLNL • Abs. constant at 65% for < 55o

• Absorption goes to 0 for grazing incidence• Main features don’t depend critically on the

exact shape

What Else Is Going On With Cones?

• Titan Laser PrepulseTitan Laser Prepulse• 3 ns fluorescence pedestal• 1x10-4 energy contrast • 1x10-8 intensity contrast - 14mJ energy in prepulse

• LLNL Titan slab shots w/ probe show preplasma -- ~40-60 LLNL Titan slab shots w/ probe show preplasma -- ~40-60 mm• Fold slab region into confined geometry of cone makes it worseFold slab region into confined geometry of cone makes it worse• Critical surface still very close to tip, but underdense out in frontCritical surface still very close to tip, but underdense out in front

1x10-8 Diagnostic artifact

ASE/Fluorescence

What About Preplasma Inside?

• ““Baton Effect”Baton Effect”– Sophie Baton’s measurements in open cones

– Submitted to Plasma Physics

Courtesy of Sophie Baton SB- 3rd FPPT- 03/2007- 9 Ref. imageProbe beam

At - 23 ps before main pulse

Before arrival of the main pulse, extension

of the preplasma L is ≥ 100 µm. 100 µm

With real cone => L should increase

~ 300 fs

Preplasma Will Fill cone tip

PrepulseShort Pulse

• Short Pulse deposits energy at critical surface near cone tip• Hot electrons use interior of cone for transport due to preplasma• Isolated cone is a refluxing type target

• Allows electrons to distribute energy

Focusing doesn’t really matter ….

Defocus or movement of focus does not affect the K production due to preplasma …

Cones act like a ~300 m deep bucket for energy coupling

Protons from Cone verify “bucket”

Cone Summary

• Cones DO guide light …. but…..Cones DO guide light …. but…..– Wall absorption deposits energy up to 50 m from tip– Current absorption numbers reflected energy not small

• Preplasma fills cone tip region Preplasma fills cone tip region – Baton’s work showed it– Preplasma perhaps provides transport path for electrons– Our cones distribute energy ~300 m from tip w/ 14 mJ

prepulse– FI scale ignition beams

• OMEGA EP 250 mJ• NIF-ARC 1.2 J Potential Trouble?

CONSEQUENCES FOR FAST IGNITIONCONSEQUENCES FOR FAST IGNITION

• Must revisit the fundamental issues of electron sourceMust revisit the fundamental issues of electron source– Must understand conversion efficiency– Must understand idea of Thot

– Look at FI relevant energy region 1-2 MeV• Understanding isolated conesUnderstanding isolated cones

– Seem to get more absorption than foils– Preplasma will be present– Will the electrons have the correct energy?– Will the electrons be directed correctly?– Do we need 2?